According to Us Weekly, the surrogate is already three months pregnant.
The surrogate, a California woman referred by an agency, is in her late 20s, has children of her own and has served as a surrogate before, Us Weekly reported.
According to TMZ, Kardashian and West will pay the surrogate $45,000 in 10 $4,500 monthly installments, and the couple paid an additional $68,850 deposit to the agency. In the event the surrogate gives birth to multiples, such as twins or triplets, she will receive $5,000 for each additional child, TMZ reported.
The surrogate signed a contract agreeing to guidelines, including no drinking, smoking or drug use; no hot tubs or saunas; no raw fish; and no handling cat litter, Us Weekly reported. She also cannot apply hair dye or drink more than one caffeinated beverage per day, according to TMZ.
Kardashian, 36, and West, 40, wed in May 2014. They welcomed daughter North West in June 2013 and son Saint West in December 2015.
In a blog post, Kardashian detailed difficulties with her first two pregnancies, in which she suffered from placenta accreta, a potentially life-threatening condition in which the placenta attaches to the wall of the womb, making it difficult for the placenta to detach from the womb when a child is born.
"My doctor had to stick his entire arm in me and detach the placenta with his hand, scraping it away from my uterus with his fingernails," Kardashian wrote in her blog post. "How disgusting and painful! My mom was crying; she had never seen anything like this before. My delivery was fairly easy, but then going through that—it was the most painful experience of my life! They gave me a second epidural but we were racing against time, so I just had to deal."
Reports that the couple had hired a surrogate surfaced in June after Kardashian discussed the possibility with family and doctors on her family's show, "Keeping Up With the Kardashians."
"If the two doctors that I trust have told me it wouldn't be safe for me to get pregnant again, I have to listen to that," said Kardashian, according to PEOPLE magazine. "Also, the best thing is I could have a new baby and have no one know and live my life for a good year before we announce it."